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  <title>UDT Control Algorithm</title>
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<h1 style="direction:ltr;"><b><font color="#808000">UDT Control Algorithm</b></font></h1>
<ul><li style="direction:ltr;"><h2>Increase</h2></li>
<ul><li style="direction:ltr;"><i><font color="#CC3300">(x) = f( B - x ) * c</i> </font></li>
<li style="direction:ltr;">	where <i>B</i> is the link capacity (Bandwidth), <i>c</i> is a constant parameter</li>
</ul><li style="direction:ltr;"><h2>Constant rate control interval (<i>SYN</i>), irrelevant to RTT</h2></li>
<ul><li style="direction:ltr;">SYN = 0.01 seconds</li>
</ul><li style="direction:ltr;"><h2>Decrease</h2></li>
<ul><li style="direction:ltr;">Randomized decrease factor</li>
<li style="direction:ltr;"><i> = 1 – (8/9)n</i></li>
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<h3>Notes:</h3>
<p style="direction:ltr;"><font color="#000000">Here is the control algorithm used in UDT. The alpha(x) is related to the difference between the network bottleneck bandwidth and the current data sending rate. As x increases, (B-x) will decrease, and we want f() to be an non-decreasing function.</font></p>
<p style="direction:ltr;"><font color="#000000"></font></p>
<p style="direction:ltr;"><font color="#000000">The UDT increase function actually uses a filter to project (B-x) onto each level of 10's power, which results a stepwise function. This is to be robust to bandwidth estimation error. I will talk about this shortly after.</font></p>
<p style="direction:ltr;"><font color="#000000"></font></p>
<p style="direction:ltr;"><font color="#000000">UDT uses a constant rate control interval in order to realize RTT fairness and reach fast convergence in long RTT links.</font></p>
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<p style="direction:ltr;"><font color="#000000">We will describe how to estimate the network bandwidth B shortly after, but now let's look at an example of how UDT increases its sending rate.</font></p>
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<p style="direction:ltr;"><font color="#000000">(x UNIT: bits/sec)</font></p>
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